AH = 32h
        DL = drive number
            0 = default, 1 = A, etc.
Return: AL = 0FFh if invalid drive number, else
        DS:BX = address of drive parameter block.

                STRUCTURE OF DOS DRIVE PARAMETER BLOCK:

DPBLOCK         STRUCT          ;OFFSET
DISK_OFFSET     DB      ?       ;  0.  drive number (0 = A, etc.)
UNIT_OFFSET     DB      ?       ;  1.  unit number within device driver
SECTOR_SIZE     DW      ?       ;  2.  number of bytes per sector
MAX_CLUSTER     DB      ?       ;  4.  largest sector number in cluster
                                ;      add one for number of sectors/cluster
LOG2_SECTORS    DB      ?       ;  5.  log base two of the cluster size
RESERVED        DW      ?       ;  6.  number of reserved (boot) sectors
FAT_COUNT       DB      ?       ;  8.  number of copies of the FAT
ROOT_COUNT      DW      ?       ;  9.  number of root directory entries
DATA_START      DW      ?       ; 11.  first data sector on medium
MAX_NUMBER      DW      ?       ; 13.  largest possible cluster number
                                ;      subtract one for number of data clusters
FAT_SECTORS     DB      ?       ; 15.  number of sectors in one FAT copy
ROOT_START      DW      ?       ; 16.  first sector of root directory
DEVICE_ADDR     DD      ?       ; 18.  address of device driver for this drive
DESCRIPTOR      DB      ?       ; 22.  media descriptor byte for medium
VALID_BYTE      DB      ?       ; 23.  0FFh indicates block must be rebuilt
NEXT_BLOCK      DD      ?       ; 24.  address of next device block in list

;       FROM THIS POINT ON, DOS 3 DIFFERS FROM 2:

                IF      DOS2
DIR_START       DW      ?       ; 28.  starting cluster of current directory
                                ;      zero indicates the root directory
PATH_NAME       DB      64 DUP (?)
                                ; 30.  ASCIIZ current directory path string
                ELSE    DOS3
;               on my XT, this was always:
                DW      0
                DW      0FFFFh
                ENDIF
DPBLOCK         ENDS
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